Nano Physics
Transcript of Nano Physics
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A Day in the Life of a Nanoparticle
Or how I learned to not sunburn and still look good.
http://www.wsu.edu/~jtd/physunder/physun2.jpg http://www.rdecom.army.mil/rdemagazine/200402/images/itl_arl_particles.jpg
http://media.photobucket.com/imag
e/sunscreen%20and%20nanoparticles/vivawoman/badger-spf30-sunscreen.jpg
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Nanoparticle Uses
Sunscreens
Make-up
Automotive Paint
Sporting Goods
anti-bacterial
Hong Dong
FE-SEM: Zeiss(1550)-Clark
This image shows electrospun nylon 6 nanofibers decorated with surface bound Ag nanoparticles.Immersing nylon 6 nanofibers into Ag colloidal solution with pH 5, Ag nanoparticles were assembledonto nylon 6 nanofibers via interaction between nylon 6 and protection groups of Ag nanoparticles.Future applications include antibacterial filtration.
Fiber Science and Apparel Design
Advisor Juan Hinestroza
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When physicists first began investigating the structure ofatoms in the early 1900s, they uncovered a strange newworld. The subatomic particles they found -- electrons,protons, and neutrons -- seemed to behave according toa completely different set of laws than those governingour everyday world.
Then, in the late 1920s, a team of young physicists ledby Niels Bohr introduced a theory that explained thebehavior of atoms and their particles. Not surprisingly,
the theory, called quantum mechanics, was as bizarreas the world it attempted to explain.
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Rather than identifying precisely where an electron shouldbe, for example, quantum mechanics predicts only theprobability of finding that electron in one place or another.
This description of unpredictability at the atomic level --
indeed, at any level -- was completely unacceptable toEinstein; it flew in the face of everything he believed, anddirectly contradicted his orderly theories of the universe.
Despite Einstein's disapproval, quantum mechanics has onlygrown in acceptance as a theory.
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The Quantum CafMichael Greene
http://www.pbs.org/wgbh/nova/programs/ht/qt/3012_qd_05.html
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Opinions on quantum mechanicsI think it is safe to say that no
one understands quantummechanics. Do not keep sayingto yourself, if you can possiblyavoid it, But how can it be likethat? because you will getdown the drain into a blindalley from which nobody has yetescaped. Nobody knows how itcan be like that.
- Richard Feynman
Richard Feynman (1918-1988)
Those who are not shockedwhen they first come acrossquantum mechanics cannot
possibly have understood it.
- Niels Bohr
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# How did our understanding of the atomchange in the 1920s?
# How did quantum mechanics contradictEinstein's view of physics? What did Einsteinmean when he said, "God does not throwdice"?
# What are some of the "bizarre" things thatquantum mechanics predicts?
Important Questions
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The Birth of Modern Physics
Classical Physics of the 1890s
The Kinetic Theory of Gases
Waves and Particles
Conservation Laws and Fundamental Forces
The Atomic Theory of Matter
Outstanding Problems of 1895 and NewHorizons
The more impo rtant fundamental laws and facts of physical sc ience have all
been discov ered, and these are now so f i rm ly establ ished that the possib i l i ty
of their ever being supp lanted in con sequence of new disco ver ies isexceedingly remote Our future discoveries must be looked for in the sixthplace of decimals. - Albert A. Michelson, 1894
There is nothin g new to b e discovered in phys ics now. Al l that remains is more
and mo re precise measurement. - Lord Kelvin, 1900
James Clerk Maxwell
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Classical Physics of the 1890s
Mechanics
Thermodynamics
Electromagnetism
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0 0B
t
BE
t
0B
0/E q
Electromagnetism culminatedwith Maxwells Equations
Gausss law :(electric field)
Gausss law: (magnetic field)
Faradays law:
Ampres law:
James Clerk Maxwell(1831-1879)
in the presence ofonly stationarycharges.
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Faraday saw the World in a new way!
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The Nature of Light
Newton promoted the corpuscular(particle) theory
Particles of light travel in straight linesor rays
Explained sharp shadows
Explained reflection and refraction
"I procured me a triangular glass prism to trytherewith the celebrated phenomena ofcolours." (Newton, 1665)
Newton in action
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The Nature of Light
Huygens promoted the wave theory.
He explained polarization,reflection, refraction, and double
refraction.
Double refraction
Christiaan Huygens(1629-1695)
He realized that light propagates as awave from the point of origin.
He realized that light slowed down on
entering dense media.
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Diffraction confirmed light to be a wave.
Diffraction patterns
One slit
Two slits
While scientists of Newtons time thought shadows were sharp, Youngs two -slit
experiment could only be explained bylight behaving as a wave. Fresnel
developed an accurate theory of diffraction
in the early 19th
century.
Augustin Fresnel
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Waves can interfere.
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Maxwell strove to prove his Mentor correct
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Light waves were found to be solutions toMaxwells Equations.
All electromagnetic wavestravel in a vacuum witha speed c given by:
infrared X-rayUVvisible
wavelength (nm)
microwave
radio
105106
gamma-ray
The electromagnetic spectrum is vast.
where 0
and 0
are the permeability and permittivity of free space
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Light is an electromagnetic wave.
The electric (E) and magnetic (B) fields are in phase.
The electric field, the magnetic field, and the propagation direction areall perpendicular.
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Triumph of Classical Physics:
The Conservation Laws
Conservation ofenergy: The sum of energy(in all its forms) is conserved (does not
change) in all interactions.
Conservation oflinear momentum: In the
absence of external forces, linearmomentum is conserved in all interactions.
Conservation ofangular momentum: In the
absence of external torque, angular
momentum is conserved in all interactions.
Conservation ofcharge: Electric charge is
conserved in all interactions.
These laws remain
the key to interpreting
even particle physics
experiments today.
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Problems in 19th-century physics
In a speech to the Royal Institution in 1900, Lord Kelvin himselfdescribed two dark clouds on the horizon of physics:
The question of the existenceof an electro-magnetic
mediumreferred to asether or aether.
The failure of classical
physics to explain blackbodyradiation.
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The Ultraviolet Catastrophe
Lord Rayleigh used the classical theories of electromagnetism
and thermodynamics to show that the blackbody spectrumshould be:
This worked at longer wavelengths but deviates badly at short ones.This problem became known as the ultraviolet catastrophe and was one of
the many effects classical physics couldnt explain.
Rayleigh-Jeans Formula
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More problems: discrete spectral lines
Wavelength
Emission
spectrafrom
gases ofhot
atoms.
For reasons then unknown, atomic gases emitted only certain narrow
frequencies, unique to each atomic species.
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Additional discoveries in 1895-7 contributed to thecomplications.
X-rays (Roentgen)
Radioactivity (Becquerel)
Electron (Thomson)
Zeeman effect
Roentgens x -ray
image of his wifes hand (with her wedding ring)
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The Beginnings of Modern Physics
These new discoveries and themany resulting complicationsrequired a massive revision offundamental physicalassumptions.
The introduction (~1900) of themodern theories ofspecialrelativity and quantummechanics became thestarting point of this most
fascinating revision. Generalrelativity (~1915) continued it.
Log(size)
Speed
0
c
19th-
centuryphysics
Generalrelativity
Q
uantumm
echanics
Special
relativity
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Triumph of Classical Physics:
The Conservation Laws
Conservation ofenergy: The sum of energy
(in all its forms) is conserved (does not
change) in all interactions.
Conservation oflinear momentum: In theabsence of external forces, linear
momentum is conserved in all interactions.
Conservation ofangular momentum: In the
absence of external torque, angularmomentum is conserved in all interactions.
Conservation ofcharge: Electric charge is
conserved in all interactions.
These laws remain
the key to interpreting
even particle physics
experiments today.
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For our sunscreen to work we will need to lookat an experiment designed to determine howtightly bound electrons are to a surface.
This requires coming up with Planck's Constant.
This also requires the determination of the work Function.
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Work function experiment.
http://www.walter-fendt.de/ph11e/photoeffect.htm
Workfunction for ZnO is ~4.5
Wh i Q Ph i ?
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What is Quantum Physics? Quantum physics is a branch of Science
that deals with discrete, indivisible units of
energy called quanta as described byQuantum Theory.
There are five main ideas represented in
Quantum Theory which are:
1. Energy is not continuous, but comes in
small, but discrete units.2. The elementary particles behave both
like particles and like waves.
3. The movement of these particles is
inherently random.
4. It is physically impossible to know both
the position and momentum of a particle at
any instant in time so that the more accurate
the measurement of one is, the more
inaccurate the measure of the other is.
5. The atomic world is NOTHING like the
world we live in.
Structure of the Atom
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The Atomic Models of Thomson andRutherford
Rutherford Scattering
The Classic Atomic Model
The Bohr Model of the Hydrogen Atom
Successes & Failures of the Bohr Model
Characteristic X-Ray Spectra and Atomic
Number
Atomic Excitation by Electrons
Structure of the Atom
The opposite of a correct statement is a false statement. But the opposite of a
profound truth may well be another profound truth.
An expert is a person who has made all the mistakes that can be made in a very
narrow field.
Never express yourself more clearly than you are able to think.
Prediction is very difficult, especially about the future.
- Niels Bohr
Niels Bohr (1885-1962)
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Structure of the Atom
Evidence in 1900 indicated thatthe atom was not a fundamental unit:
1) There seemed to be too many kinds
of atoms, each belonging to a distinct chemical
element (way more than earth, air, water, and fire!).
2) Atoms and electromagnetic phenomena were intimately
related (magnetic materials; insulators vs. conductors;
different emission spectra).
3) Elements combine with some elements but not with others,a characteristic that hinted at an internal atomic structure
(valence).
4) The discoveries of radioactivity, x rays, and the electron (all
seemed to involve atoms breaking apart in some way).
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Knowledge of atoms in 1900
Electrons (discovered in
1897) carried the negative
charge.
Electrons were very light,
even compared to the atom.
Protons had not yet been
discovered, but clearly
positive charge had to bepresent to achieve charge
neutrality.
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In Thomsons view, when the atom was heated, the electrons could
vibrate about their equilibrium positions, thus producing
electromagnetic radiation.
Unfortunately, Thomson couldnt explain spectra with this model.
Thomsons
Atomic Model
Thomsons plum-pudding
model of the atom had the
positive charges spread
uniformly throughout a
sphere the size of the atom,
with electrons embedded in
the uniform background.
Experiments of Rutherford Geiger and
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Experiments of Rutherford, Geiger and
Marsden
Rutherford, Geiger, and Marsden
conceived a new technique for
investigating the structure of
matter by scattering a particles
from atoms.
Experiments of Rutherford Geiger and
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Experiments of Rutherford, Geiger and
Marsden 2
Geiger showed that many a particles were scattered from thin
gold-leaf targets at backward angles greater than 90.
Rutherfords Atomic Model
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Experimental results
were not consistent with
Thomsons atomic model.
Rutherford proposed thatan atom has a positively
charged core (nucleus)
surrounded by the
negative electrons.
Geiger and Marsden
confirmed the idea in
1913.
Rutherfords Atomic Model
Ernest Rutherford
(1871-1937)
The Classical Atomic Model
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2 2
2
0
1 v
4e
e mF
r r
The Classical Atomic Model
Consider an atom as a planetary system.
The Newtons 2nd Law force of attraction on
the electron by the nucleus is:
where v is the tangential velocity of the
electron:
The total energy is then:
0
v4
e
mr
221 1
2 2
0
v4
eK m
r
This is negative, so
the system is bound,
which is good.
The Planetary Model is Doomed
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The Planetary Model is Doomed
From classical E&M theory, an accelerated electric charge radiates
energy (electromagnetic radiation), which means the total energy
must decrease. So theradius r must decrease!!
Physics had reached a turning point in 1900 with Plancks
hypothesis of the quantum behavior of radiation, so a radical
solution would be considered possible.
Electroncrashes
into the
nucleus!?
The Bohr Model of the Hydrogen Atom
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The Bohr Model of the Hydrogen Atom
Bohrs general assumptions:
1. Stationary states, in which orbiting
electrons do not radiate energy, exist in
atoms and have well-defined energies,
En. Transitions can occur between them,
yielding light of energy:
E=En En = h
2. Classical laws of physics do not apply
to transitions between stationary states,
but they do apply elsewhere.
3. The angular momentum of the nth state is:
where n is called the Principal Quantum Number.
n
n = 1
n = 3
n = 2
Angular
momentum is
quantized!
Consequences of the Bohr Model
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Consequences of the Bohr Model
The angular momentum is:
nrmL v
0
v4
e
mr
mrn /v
04
2 2 2
2 2
n e
m r mr But: So:
Solving forrn:2
0nr n a
So the velocity is:
00
4 2
2ame
where:
a0is called the Bohr radius. Its the diameter of the Hydrogen
atom (in its lowest-energy, or ground, state).
a0
Bohr Radius
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Bohr Radius
The Bohr radius,
is the radius of the unexcited hydrogen atom and is equal to:
The ground state Hydrogen atom diameter is:
00
4 2
2a
me
/
The Hydrogen
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y g
Atom Energies
So the energies of the stationary
states are:
whereE0 = 13.6 eV.
r
eE
0
2
8
04 2 2
n 2
nr
me
Use the classicalresult for the
energy:
and:
En = E0/n2or:
The Hydrogen Atom
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The Hydrogen Atom
Emission of light occurs when the atom is in an excited state
and decays to a lower energy state (nu n).
uh E E
1 h
c hc
R is theRydberg constant.
where
is the frequency of a photon.
3
0(4 )
4
2
meR
c
Transitions
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Transitions
in the
HydrogenAtom
The atom will remain
in the excited state
for a short time
before emitting a
photon and returning
to a lower stationary
state. In equilibrium,all hydrogen atoms
exist in n = 1.
Characteristic X-Ray
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Shells have letter names:
K shell forn = 1
L shell forn = 2
The atom is most stable in its
ground state.
When it occurs in a heavy atom, the radiation emitted is an x-ray.
It has the energyE(x-ray) =Eu E.
Characteristic X Ray
Spectra and Atomic
Number
An electron from highershells will fill the inner-shell vacancy at lower energy.
Th C d
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The Correspondence
Principle
In the limits where classical and
quantum theories should agree,
the quantum theory must reducethe classical result.
Bohrs correspondence principle
is rather obvious:
Successes and Failures of the Bohr
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Model
The electron and
hydrogen nucleus
actually revolve
about their mutual
center of mass.
The electron mass is replaced
by its reduced mass:
The Rydberg constant for infinite nuclear mass,R, is replaced byR.
Success:
Limitations of the
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Limitations of the
Bohr Model
Works only for single-electron (hydrogenic) atoms.
Could not account for the intensities or the fine structure of
the spectral lines (for example, in magnetic fields).
Could not explain the binding of atoms into molecules.
Failures:
The Bohr model was a great
step in the new quantum
theory, but it had its limitations.